Efficient CNN-based solution for video frame interpolation

ABSTRACT

A system of convolutional neural networks (CNNs) that synthesize middle non-existing frames from pairs of input frames includes a coarse CNN that receives a pair of images acquired at consecutive points of time, a registration module, a refinement CNN, an adder, and a motion-compensated frame interpolation (MC-FI) module. The coarse CNN outputs from the pair of images a previous feature map, a next feature map, a coarse interpolated motion vector field (IMVF) and an occlusion map, the registration module uses the coarse IMVF to warp the previous and next feature maps to be aligned with pixel locations of the IMVF frame, and outputs registered previous and next feature maps, the refinement CNN uses the registered previous and next feature maps to correct the coarse IMVF, and the adder sums the coarse IMVF with the correction and outputs a final IMVF.

TECHNICAL FIELD

Embodiments of the present disclosure are directed to methods for videoprocessing

DISCUSSION OF THE RELATED ART

In video frame interpolation (VFI), middle non-existing frames aresynthesized from the original input frames. A classical applicationrequiring VFI is frame rate up-conversion for handling issues likedisplay motion blur and judder in LED/LC displays. Other applicationsinclude frame recovery in video coding and streaming, slow motioneffects and novel view synthesis.

Conventional approaches to VFI, which include carefully hand engineeredmodules, typically include the following steps: bi-directional motionestimation, motion interpolation and occlusion reasoning, andmotion-compensated frame interpolation (MC-FI). Such approaches areprone to various artifacts, such as halos, ghosts and break-ups due toinsufficient quality of any of the components mentioned above.

In the past few years, deep learning and specifically convolutionalneural networks (CNNs) have emerged as a leading method for numerousimage processing and computer vision tasks. VFI processes can benefitfrom the use of these cutting-edge techniques, such as replacing some ofthe steps in the VFI's algorithmic flow with one or more pre-trainedCNNs.

SUMMARY

Exemplary embodiments of the present disclosure are directed to systemsand methods for synthesizing middle non-existing frames from pairs ofinput frames in a given video. In contrast to conventional approachesfor VFI, embodiments of the disclosure focus on designing a CNN-basedframework that retains simple hand-crafted modules and replaces moresophisticated components by CNNs which have been pre-trained on a largeset of examples. Methods according to embodiments of the disclosure canreduce the level of halo, ghost and break-up artifacts, which aretypical for conventional solutions. Moreover, by choosing awell-balanced mixture of conventional and CNN-based components, a methodaccording to an embodiment of the disclosure can be efficientlyimplemented on various platforms, ranging from GPUs, through neuralprocessors, to dedicated hardware.

According to an embodiment of the disclosure, there is provided a systemthat uses convolutional neural networks (CNNs) to synthesize middlenon-existing frames from pairs of input frames in a given video,comprising a coarse convolutional neural network (CNN) that receives apair of images acquired at consecutive points of time, wherein the pairof images includes a previous image and a next image, a registrationmodule connected to the coarse CNN, a refinement CNN connected to theregistration module and the coarse CNN, an adder connected to therefinement CNN and the coarse CNN; and a motion-compensated frameinterpolation (MC-FI) module connected to the adder and the coarse CNN.The coarse CNN outputs a previous feature map and a next feature mapfrom the previous image and the next image, a coarse interpolated motionvector field (IMVF) and an occlusion map from the pair of images, theregistration module uses the coarse IMVF to warp the previous and nextfeature maps to be aligned with pixel locations of the IMVF frame, andoutputs a registered previous feature map and a registered next featuremap, the refinement CNN uses the registered previous feature map and aregistered next feature map to correct the coarse IMVF, and the addersums the coarse IMVF with the correction to the IMVF and outputs a finalIMVF.

According to a further embodiment of the disclosure, themotion-compensated frame interpolation (MC-FI) module generates aninterpolated frame corresponding to a time between the time points ofthe previous frame and the next frame by warping the previous image andthe next image using the final IMVF and performing a weighted blendingof the warped previous and next images using occlusion weights from theocclusion map.

According to a further embodiment of the disclosure, the coarse CNNreceives the pair of images in a plurality of resolution levels. Thecoarse CNN includes a feature extraction sub-network that generates apair of feature maps that correspond to each image of the pair of imagesat each level of resolution, an encoder-decoder sub-network thatconcatenates the pair of feature maps at each level of resolution into asingle feature map and processes the single feature map to produce a newfeature map with downscaled spatial resolution, a fusion sub-networkthat merges the new single feature maps at each level of resolution intoa single merged feature map by performing a weighted average of thefeature maps for each level of resolution where the weights are learnedin a training phase and differ for each pixel, and an estimationsub-network that outputs horizontal and vertical components of thecoarse IMVF and an occlusion map. The feature extraction sub-networkincludes Siamese layers.

According to a further embodiment of the disclosure, the estimationsub-network includes a horizontal sub-module, a vertical sub-module andan occlusion map sub-module, where each sub-module receives the mergedfeature map output from the fusion sub-network. The horizontal andvertical sub-modules respectively output a horizontal probability mapand vertical probability map with S probability values per pixel in eachprobability map. Each probability value represents a probability for amotion vector to be one of S displacement values for that pixel. Thehorizontal and vertical sub-modules respectively calculate a firstmoment of the probability values for each pixel to determine expectedhorizontal and vertical components for each pixel, where the pairs ofexpected horizontal and vertical components for each pixel comprise thecoarse IMVF.

According to a further embodiment of the disclosure, the occlusion mapsub-module outputs the occlusion map which includes per-pixel weightsfor performing a weighted average between the previous image and thenext image.

According to a further embodiment of the disclosure, the refinement CNNincludes an encoder-decoder sub-network that concatenates the registeredprevious feature map and the registered next feature map and outputs anew set of feature maps with spatial resolution resized with respect toa full resolution of the previous image and the next image, and anestimation sub-network that estimates corrections to the horizontal andvertical components of the coarse IMVF for each block in the registerednext and previous feature maps to output the corrected IMVF.

According to a further embodiment of the disclosure, the estimationsub-network includes a horizontal sub-module and a vertical sub-module.The horizontal and vertical sub-modules respectively output a horizontalprobability map and vertical probability map with S probability valuesper pixel in each probability map, where each probability valuerepresents a probability for a motion vector to be one of S displacementvalues for that pixel. The horizontal and vertical sub-modulesrespectively calculate a first moment of the probability values for eachpixel to determine expected horizontal and vertical components for eachpixel, where the pairs of expected horizontal and vertical componentsfor each pixel comprise the correction to the IMVF.

According to another embodiment of the disclosure, there is provided amethod of using convolutional neural networks (CNNs) to synthesizemiddle non-existing frames from pairs of input frames in a given video,including receiving a pyramid representation of a pair of consecutiveinput frames, wherein the pair of consecutive input frames includes aprevious image and a next image, wherein the pyramid representationincludes a plurality of pairs of input frames, each at a differentspatial resolution level; generating a pair of feature maps from eachresolution level of the pyramid representation and estimating a coarseinterpolated motion vector field (IMVF) and an occlusion map from eachpair of feature maps; registering pairs of feature maps at the sameresolution level according to the coarse IMVF and the occlusion map bywarping each feature map of the pair of feature maps to be aligned withpixel locations of the coarse IMVF and outputting a registered previousfeature map and a registered next feature map; correcting the coarseIMVF using the registered previous feature map and the registered nextfeature map to generate a correction to the IMVF; adding the correctionto the IMVF to the coarse IMVF to generate a refined IMVF; and producinga synthesized middle frame from the pair of consecutive input frames,the refined IMVF and the occlusion map.

According to a further embodiment of the disclosure, generating a pairof feature maps includes generating a pair of features maps for each ofthe plurality of pairs of input frames at each spatial resolution, whereeach pair of features maps has a spatial resolution downscaled withrespect to a resolution of the pair of input frames; concatenating thefeature maps at each resolution level and processing the concatenatedfeature maps to generate a new set of feature maps with downscaledspatial resolution with respect to a resolution of the pair ofconsecutive input frames, merging the new set of feature maps for allspatial resolution levels into a single merged feature map by performinga weighted average of the feature maps for each level of resolutionwherein the weights are learned in a training phase and differ for eachpixel; and estimating for each block in the merged feature maphorizontal and vertical components of the coarse IMVF, and an occlusionmap, where the occlusion map includes per-pixel weights for performing aweighted average between the previous image and the next image.

According to a further embodiment of the disclosure, estimatinghorizontal and vertical components of the coarse IMVF includesgenerating a horizontal probability map and vertical probability mapwith S probability values per pixel in each probability map, where eachprobability value represents a probability for a motion vector to be oneof S displacement values for that pixel, and calculating a first momentof the probability values for each pixel to determine expectedhorizontal and vertical components for each pixel, wherein the pairs ofexpected horizontal and vertical components for each pixel comprise thecoarse IMVF.

According to a further embodiment of the disclosure, the coarse IMVFincludes two 2D maps of horizontal and vertical shifts directed from theIMVF to the next frame, and registering pairs of feature maps includesusing the opposite values of these shifts to warp features from theprevious frame to locations of an output frame, and using the horizontaland vertical shifts as is to warp the input features from the next frameto the locations of the output frame.

According to a further embodiment of the disclosure, registering pairsof feature maps further includes, when the registered previous featuremap has an occluded region, replacing the occluded region in theregistered previous feature map with a corresponding region of theregistered next feature map, and when the registered next feature maphas an occluded region, replacing the occluded region in the registerednext feature map with a corresponding region of the registered previousfeature map.

According to a further embodiment of the disclosure, correcting thecoarse IMVF includes concatenating features of the registered previousfeature map and the registered next feature map and processing theconcatenated feature maps to generate a new set of feature maps withspatial resolution resized with respect to the full resolution; andestimating for each block in the coarse IMVF corrections to thehorizontal and vertical components of the coarse IMVF to generate acorrection to the IMVF.

According to a further embodiment of the disclosure, producing asynthesized middle frame includes warping the pair of consecutive inputframes according to the refined IMVF; and performing a weighted blendingof the pair of warped images using the occlusion weights to generate thesynthesized middle frame.

According to a further embodiment of the disclosure, the method includesremoving blockiness artifacts from motion boundaries by performing aweighted averaging of interpolated pixels in the synthesized middleframe using estimates from neighboring blocks.

According to another embodiment of the disclosure, there is provided anon-transitory program storage device readable by a computer, tangiblyembodying a program of instructions executed by the computer to performthe method steps for using convolutional neural networks (CNNs) tosynthesize middle non-existing frames from pairs of input frames in agiven video.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a pair of consecutive input frames, according toembodiments of the disclosure.

FIG. 2 illustrates a pair of consecutive input frames with complexmotion, according to embodiments of the disclosure.

FIG. 3 illustrates an example of a coarse CNN with 3 pyramid levels,according to embodiments of the disclosure.

FIG. 4 illustrates an occlusion-aware registration module, according toembodiments of the disclosure.

FIG. 5 shows an example of a Refinement CNN, according to embodiments ofthe disclosure.

FIG. 6 shows an overview of a CNN-based framework for VFI, according toembodiments of the disclosure.

FIG. 7 illustrates results of a hierarchical Coarse CNN and a MC-FImodule on the pair of frames from FIG. 1, according to embodiments ofthe disclosure.

FIG. 8 illustrates results of a Coarse CNN and a Refinement CNN on thepair of frames from FIG. 2, according to embodiments of the disclosure.

FIG. 9 illustrates results of a CNN-based VFI solution on a pair offrames from FIG. 2, according to embodiments of the disclosure.

FIG. 10 is a block diagram of a system that implements a method forusing CNNs to synthesize middle non-existing frames from pairs of inputframes in a given video, according to an embodiment of the disclosure.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Exemplary embodiments of the disclosure as described herein generallyprovide systems and methods for using CNNs to synthesize middlenon-existing frames from pairs of input frames in a given video. Whileembodiments are susceptible to various modifications and alternativeforms, specific embodiments thereof are shown by way of example in thedrawings and will herein be described in detail. It should beunderstood, however, that there is no intent to limit the disclosure tothe particular forms disclosed, but on the contrary, the disclosure isto cover all modifications, equivalents, and alternatives falling withinthe spirit and scope of the disclosure.

FIG. 1 depicts a pair of consecutive input frames, in which the previousframe is on top and the next frame is on the bottom. In this example,the background remains static, whereas the left hand and the rest of theupper-body move each with a different motion vector.

Basic Assumptions

According to embodiments of the disclosure, linear motion between a pairof consecutive input frames is assumed. Specifically, this means thatthe motion from every pixel location in the estimated middle frame tothe previous frame equals minus the motion from this location to thenext frame.

Task Definition

Methods according to embodiments of the disclosure decompose the task ofsynthesizing the middle frame from a pair of two consecutive inputframes into three steps:

-   -   1) Estimating the motion from each pixel location in the        synthesized frame to its corresponding locations in each of the        input frames. The set of motion vectors from all such pixel        locations is regarded as the interpolated motion vector field        (IMVF).    -   2) Classifying each pixel location in the synthesized frame into        one of the following categories: ‘closing’, ‘opening’ or        ‘non-occluded’. The first means that this pixel location can be        found only in the previous input frame, the second means that it        can be found only in the next frame and the latter means that it        can be found in both input frames. The map of these        classification categories is regarded as the occlusion map.    -   3) Motion compensated warping of the input frames and weighted        blending of the warped inputs according to the occlusion map,        which serve as local weights for a blending procedure.        CNN-Based FrameWork for VFI

A system according to embodiments of the disclosure includes two CNNsand two conventional modules to handle registration andmotion-compensated frame interpolation (MC-FI). A first, coarse CNNaccording to an embodiment estimates a coarse IMVF and occlusion map foreach K×K block in the output interpolated frame. These estimates areused by a registration module to create a registered feature map thatserve as input to a second CNN according to an embodiment, which refinesthe IMVF. The refined IMVF and the occlusion map are input into an MC-FImodule that produces the interpolated middle frame.

Each of the two CNNs according to an embodiment plays a different role.A coarse CNN according to an embodiment can capture the most dominantmotions in the scene, which are typically a composition of global motionand a set of simple rigid local motions. The coarse CNN can alsodiscover occluded regions, mainly on motion boundaries. Using the coarseCNN's estimations for the IMVF and occlusion map, the input featuresextracted from these frames are warped. After this warping, inaccuraciesand residual motion may still remain. A second, refinement CNN accordingto an embodiment operates on the warped input features and can correctinaccuracies and capture secondary motions.

According to embodiments, breaking IMVF estimation into two steps isuseful for scenes with complex motion. An example of a pair of inputframes with complex motion is shown in FIG. 2, in which the previousframe is at the top and the next frame is at the bottom. The car locatedclosest to the camera is moving to the right, its wheels are alsospinning, while another car located further away from the camera ismoving in a different direction. A coarse CNN according to an embodimentcan capture the camera motion as well as the local motion of the carlocated closest to the camera. However, it is challenging to accuratelycapture at the same time the motion of the spinning wheels. This is thesituation where a Refinement CNN according to an embodiment plays arole.

According to embodiment, the basic building blocks for constructing bothCNNs include convolution, average pooling, bilinear up-sampling,concatenation, and soft-max layers; all are a common practice in thedeep learning field. Using these building blocks, one can design thefollowing types of sub-networks:

1. Feature extraction;

2. Encoder-Decoder,

3. Feature fusion; and

4. Estimation.

Each sub-network is constructed of specific layers, which include thefollowing types:

(i) Convolution;

(ii) Average Pooling;

(iii) Concatenation;

(iv) Bilinear up-sampling;

(v) SoftMax; and

(vi) Center-of-Mass.

The first five types are a very common practice in the deep learningfield. Note that each convolution layer that is not followed by aSoftMax layer comprises a non-linearity, typically a rectified linearunit activation. The last type is a linear operation in which the firstmoment of a given distribution is computed. The two CNNs according toembodiments are composed of some or all of these types of sub-networks.Each convolution layer in a neural network has its own set ofparameters. The parameters of a convolution layer in a neural networkinclude weights and bases. In a convolution layer the weights correspondto the coefficients of the 3D convolution kernel, and the biases are aset of offsets added to the results of a 3D convolution operation. Forexample, if a convolution layer uses a 3×3 filter support size, a pixeland the 3×3 grid of pixels around it are considered. Each one of thesepixels is multiplied by a weight value and then all these weights aresummed up. Then the bias is added and the value of one output featureproduced. This is repeated for all pixel locations and for all outputchannels, where a different 3×3 filter and bias may be applied for eachoutput channel. The weights and biases of the convolutional layers arelearned when training the CNN. To learn the parameters of each CNN, aCNN is trained together with a given MC-FI module, so that an outputframe is generated. This allows training the CNN in an end-to-endfashion using a dataset of frame triplets, where in each triplet themiddle frame serves as ground truth for frame synthesis from the twoother frames. Specifically, this means that ground truth for VFI can bereadily obtained by sub-sampling videos.

Coarse CNN

A hierarchical coarse CNN according to an embodiment receives a pair ofinput frames at several resolutions, e.g.: level 0—full-resolution,level 1—downscaled by a factor of 2, Level 2—downscaled by a factor of4, and so on. See FIG. 3 for an illustration of the image pyramids. Togenerate the image pyramid for each input frame, standard image resizingoperations were used with factors of 2 (for Level 1) and 4 (for Level 2)per axis. These resizing operations are based on bilinear or bicubicinterpolations. Each input goes through a same pair of layers with thesame set of layer parameters. This pair of layers is typically referredto as Siamese layers. The Siamese layers produce a set of features mapsper input, for an output of N values per pixel, with a factor of Mreduction in the number of pixels per axis with respect to the inputresolution. An exemplary, non-limiting value of N is N=25. As with therest of the network, in this case the weights and biases are learned.Low-level features extracted from the Siamese layers of a CNN typicallyinclude strong responses on image edges and color representations.

According to embodiments, the encoder-decoder sub-network is trained toextract features with increasing high-level context when moving frominput to output. The output of the encoder-decoder is a 3D image, inwhich each spatial entry in this image is a high-dimensionalrepresentation. Each pair of feature maps taken from the same pyramidlevel in both input frames are concatenated and processed byencoder-decoder sub-network layers, resulting in a new set of featuremaps with spatial resolution downscaled by L with respect to the fullresolution. By concatenating is meant appending N feature channels fromthe second input frame to the N feature channels of the first inputframe, yielding 2N feature channels overall. The 2N feature channels arepassed as input to the encoder-decoder layers which yields an outputwith J values (channels) per pixel with factor of L reduction in thenumber of pixels per axis with respect to the input image. For example,in some embodiments, J=2N. The processing performed by a encoder-decodersub-network is a deep cascade of convolution, average pooling andup-sampling layers. Each convolution layer is also followed by anonlinearity unit, such as a rectified linear unit. Combined together,this sub-network according to an embodiment learns how to extractfeatures with increasing high-level context as the processing moves frominput to output. The output of the encoder-decoder is a 3D image, whereeach spatial entry in this image is a high-dimensional feature vector.

Then, according to an embodiment, the outputs of the all decoders aremerged into a coarse IMVF in a locally adaptive fashion using a fusionsub-network. According to an embodiment, there are three encoder-decodersubmodules, one for each level of resolution, and each yields an outputwith the same size: WL×H/L×2N, where W and H are the width and height ofthe input frames in pixels. The fusion submodule takes these threeoutputs and combines them into a single output by performing a weightedaverage of the three inputs. The weights of this average are adaptive inthe sense that they are learned and local in the sense that they aredifferent for every pixel.

According to an embodiment, non-limiting values of L and M are L=8 andM=4. These values are basic properties of the architecture of a network,and are set by the number and structure of layers that change thespatial resolution within the CNN.

According to embodiments, the merged decoder output is input intoestimation sub-network layers that estimate for each K×K block in theoutput image the horizontal and vertical components of the coarse IMVF,as well as the occlusion map. For example, in some embodiments, K=L. Thecoarse estimation layers include three sub-modules: a horizontal coarseIMVF sub-module, a vertical coarse IMVF sub-module and an occlusion mapsubmodule, and each sub-module receives the output of the fusionsub-network. The horizontal and vertical coarse IMVF sub-modules eachoutput a similar structure: a coarse horizontal and vertical probabilitymap of size W/L×WL×S, i.e., S values for each pixel in the probabilitymap. Each of the S probabilities per pixel represents the probabilityfor a motion component to be one of S displacement values, e.g. forS=25, the displacements may be −96, −88, . . . 0, . . . 88, 96 in pixelunits. For example, in some embodiments, S=N. Then, the first moment(center-of-mass) of this distribution is calculated for each pixel todetermine the horizontal and vertical components of for each pixel. Thepairs of expected values of the horizontal and vertical components foreach pixel are the IMVF.

According to an embodiment, an estimation path for each of thehorizontal/vertical motion component comprises a shallow cascade ofconvolution, soft-max and center-of-mass layers. The convolution layersprocess the output of the merged decoder outputs so that it will matchthe specific estimation task in hand. The soft-max layer converts theoutputs of the last convolution layer to normalized discrete probabilityvectors, with one probability vector for each spatial location. Each ofthe motion estimation paths uses S classes, each corresponding to aspecific motion value. The center-of-mass layer computes the firstmoment of each the probability distribution and thus converts theprobability vectors into an expected motion value.

According to an embodiment, when creating a new intermediate frame, theprevious and next frames are separately registered according to theIMVF, after which they are combined into a single frame. This is doneusing the occlusion map, which comprises per-pixel weights forperforming weighted average between the previous and next frames. Theper-pixel occlusion weights are computed via a separate estimation path.Similar to the motion estimation paths, a shallow cascade of convolutionand soft-max layers is applied to the merged decoder outputs. In thisestimation path there are two classes—“covering” and “uncovering”. Theocclusion weights are computed by taking the probability of the firstclass. When the value of the occlusion map is 0.5, then the pixels fromthe previous and next frames are averaged with equal weights. When thevalue of the occlusion map is 1 only the previous frame pixel is used,and when the value of the occlusion map is 0 only the next frame pixelis used. These values of 0 and 1 are used in case of occlusions. Forexample if a ball moves between two frames, the pixels that were behindthe ball in the previous frame are now visible, and when performing theinterpolation, only some of the pixels from the next image (the nowexposed new pixels) and only some of the pixels from the previousframe—the pixels that are hidden by the advancing ball in the nextframe—are used. The occlusion map is determined by a sub-module similarto the IMVF sub-modules, but instead of obtaining S channels per pixel,only two channels are output, where the occlusion map is one of thechannels.

An example of a coarse CNN with 3 pyramid levels is shown in FIG. 3.Referring now to FIG. 3, two consecutive input frames given at threeresolutions are shown on the left. The three resolutions are indicatedby Level 0, Level 1 and Level 2. Note that this number of resolutionlevels is exemplary and non-limiting, and there may be more or fewerresolution levels in other embodiments. Each pair of input frames isinput into the Siamese layers of a feature extraction sub-network 31,which output a pair of feature maps for each pair of input frames. Thepair of features maps output by the Siamese layers are input toencoder-decoder sub-networks 32, which output processed feature mapsthat are provided to the feature fusion sub-network 33. The featurefusion sub-network 33 merges the outputs of the encoder-decodersub-networks 32 to yield the coarse IMVF and the occlusion map 34. Theestimated IMVF, overlaid on the output image, and occlusion map areshown on the right.

Occlusion-Aware Registration

According to an embodiment, a registration module receives a pair ofinput feature maps, extracted by a convolution layer of the Siamesesub-network of a given pyramid level. Using the coarse IMVF output fromthe feature fusion sub-network, the registration module warps each ofthese feature maps to be aligned with the pixel locations of the outputframe, and yields a registered previous feature map and a registerednext feature map.

According to an embodiment, an IMVF includes two 2D maps of horizontaland vertical shifts directed from the output frame to the next frame. Ina registration module these shifts are used to generate two versions ofwarped input features:

-   -   (1) The opposite values of these shifts are used to warp the        input features from the previous frame to the locations of the        output frame.    -   (2) These shifts are used “as is” to warp the input features        from the next frame to the locations of the output frame.

FIG. 4 illustrates an exemplary flow of an occlusion-aware registrationmodule according to an embodiment. Due to occlusions, some regions in anoutput frame can be synthesized only from one of the input frames. Thismeans that in the warped feature maps there are regions with very lowconfidence. To handle these regions, according to embodiments, afollowing occlusion-aware compensation can be applied. Referring to thetop row of FIG. 4, replace occluded regions (white holes) in the warpedfeature maps of the previous frame 42 a with the warped features of thenext frame at these locations 42 b to yield output frame 43 a, andreferring to the bottom row of FIG. 4, replace occluded regions in thewarped feature maps of the next frame 42 b with the warped features ofthe previous frame at these locations 42 a to yield output frame 43 b.After this compensation, the registered previous and next feature mapswill coincide on the occluded pixel location, which leaves no furtherroom for motion refinement in these locations.

Refinement CNN

An example of a refinement CNN according to an embodiment that receivesregistered features maps from one level of the image pyramids is shownin FIG. 5. A refinement CNN uses the two coarsely registered inputfeatures as inputs, concatenates them, and then processes them similarlyto the coarse CNN. Similar to the coarse CNN, a refinement CNN accordingto an embodiment includes a horizontal sub-module and a verticalsub-module. Referring to FIG. 5, a refinement CNN receives the pair ofregistered feature maps produced by the registration module. Thesefeatures are concatenated and processed by an encoder-decodersub-network 51 similar to the coarse CNN, resulting in a new set offeature maps with spatial resolution downscaled by Q with respect to thefull resolution. An exemplary, non-limiting value of Q is Q=4. Thedecoder output and the coarse IMVF is then provided to estimationsub-network layers 52 that estimate for each P×P block in the outputimages corrections to the horizontal and vertical components of thecoarse IMVF to output a refined IMVF. For example, in some embodiments,P=Q. A refined CNN according to an embodiment yields the IMVF correctionusing the coarsely registered input frames, and not directly from thecoarse IMVF. The IMVF corrections are then summed with a resized versionof the coarse IMVF to create the final, refined IMVF. The resizingoperation aligns the outputs estimated by the coarse and refinement CNNsto the same resolution. In the example where K=8, L=8, P=4 and Q=4, anup-scaling operation with factor 2 at each axis is used.

MC-FI Module

An MC-FI module according to an embodiment uses the two full resolutioninput frames, the refined IMVF and the occlusion map to create a newinterpolated frame at a given timestamp. According to an embodiment, aMC-FI module generates the output frame from the input frames using theIMVF similar to how the registration module warps each of these featuremaps, where the occlusion weights are used for blending the two inputframes. The MC-FI module first warps the input frames according to theIMVF, similar to the first step of a registration module according to anembodiment. Then the MC-FI module performs a weighted blending of thewarped images using the occlusion weights. Since the IMVF and occlusionweights are estimated per each K×K block in the output image, the MC-FImodule includes a de-blocking mechanism which removes blockinessartifacts from motion boundaries. More specifically, the MC-FI moduleperforms a weighted averaging of interpolated pixels obtained using theestimates from neighboring blocks.

Algorithmic Flow

A flow of a CNN-based framework according to an embodiment for VFI isillustrated in FIG. 6. Referring to the figure, according to anembodiment, a whole algorithmic flow includes:

-   -   1. Receiving a pair of consecutive input frames;    -   2. Generating a pyramid representation from these frames;    -   3. Providing the pairs of image pyramids to a first,        hierarchical coarse CNN 61 that outputs a pair of feature maps        for each pair of image pyramids and estimates a coarse IMVF and        an occlusion map;    -   4. Registering, by a registration module 62, pairs of feature        maps generated by coarse CNN 61 according to the IMVF and        occlusion map estimated by the coarse CNN 61;    -   5. Providing the coarse IMVF and the registered pairs of feature        maps to a second refinement CNN 63 that estimates the        corrections to the coarse IMVF;    -   6. Summing, by an adder 64, the corrections to the coarse IMVF        with an up-scaled version of the coarse IMVF to create the final        IMVF; and    -   7. Providing the original pair of input frames, along with the        final IMVF and the estimated occlusion map, to an MC-FI module        65 that produces the synthesized middle frame.

Note that: (1) the coarse CNN can receive one or more levels from theimage pyramids; and (2) the registration module and the refinement CNNcan be bypassed if fast inference is preferred over high accuracy.

Effects

Methods according to embodiments can incorporate deep learning modulesinto a VFI framework to replace some or all of the conventional handengineered components. The pre-trained neural networks can:

-   -   1. more accurately estimate occluded regions and complex motion;        and    -   2. are more robust to abrupt brightness changes.

These features can improve frame interpolation quality, thus effectivelyreducing the level of halo, ghost and break-up artifacts, typicallyobserved in standard VF methods.

In addition, methods according to embodiments can learn from examples toperform the following tasks:

-   -   1. Directly estimating the IMVF from the deep feature space        instead of computing forward and backward MVFs and then        interpolating them to a desired output grid.    -   2. Directly estimating the occlusion map from the deep feature        space instead of estimating it by hand engineered analysis of        the forward and backward MVFs.

Methods according to embodiments can balance between using pre-trainedneural networks and conventional hand engineered modules in a VFIsolution. A good balance between the two types of components, along withcareful design of the neural network architecture, can facilitate anefficient implementation of a high quality VFI on platforms such asGPUs, neural processors and dedicated hardware.

Embodiments of the disclosure can be used for various applications thatrely on VFI:

-   -   1. Frame rate up-conversion for reducing display motion blur and        judder in LED/LCD displays;    -   2. Frame recovery in video coding and streaming;    -   3. Increasing frame rate in video conference calls;    -   4. Slow motion effects; and    -   5. Synthesizing novel views.

In scenarios like (3) and (4), the motion and occlusion might be toocomplex and the image quality requirement might be too strict forconventional VF approaches to handle. A system that uses CNN-basedtechniques according to embodiments can better cope with the challengesfaced in these scenarios.

Examples

FIGS. 7-9 illustrate results obtained by a CNN-based solution for VF,according to embodiments of the disclosure.

FIG. 7 depicts the outputs produced by the hierarchical Coarse CNN andthe MC-F module according to an embodiment when applied to the pair ofinput frames shown in FIG. 1, including the estimated IMVF and occlusionmap. It also shows the middle frame synthesized by the MC-FI moduleusing these estimations. The top row shows, from left to right: theestimated horizontal and vertical components of the IMVF and theestimated occlusion map. The bottom row shows, from left to right: theestimated MVF overlaid on the synthesized frame and the estimatedocclusion map overlaid on the synthesized frame. Note that regions 72depict a ‘closing’ occlusion type, and the rest of the pixel locationsare un-occluded.

Results for the pair of input frames with complex motion shown in FIG. 2are shown next. FIG. 8 depicts the coarse IMVF obtained by a Coarse CNNthat uses only the finest level of the image pyramids (full resolution)and the estimated corrections to the IMVF produced by the RefinementCNN. The top row shows, from left to right: the horizontal and verticalcomponents of the IMVF, which were estimated by a Coarse CNN that usesonly the finest level of the image pyramids (full resolution). Thebottom row shows, from left to right: the horizontal and verticalcomponents of the delta IMVF, which were estimated by the RefinementCNN. It can be seen that the second CNN detected inaccuracies in theIMVF across the front car, and also recovered secondary motion of thewheels of this car.

FIG. 9 illustrates results of a CNN-based VFI solution on the pair offrames from FIG. 2, and displays the middle frame synthesized by twopossible choices of the suggested framework: (i) Coarse CNN+MC-FI; and(ii) Coarse CNN+Registration+Refinement CNN+MC-FI (the entire pipeline).The top row shows a synthesized frame generated by VFI solution thatincludes a Coarse CNN+MC-FI. The bottom row shows a synthesized framegenerated by an entire pipeline of a VFI solution according to anembodiment (Coarse CNN+Registration+Refinement CNN+MC-FI). Whencomparing these two frame results, it is easy to observe the qualityimprovements achieved by using a full pipeline: the text appears muchclearer, the boundaries of the car parts are sharper and the level ofdetails in the wheels is improved.

System Implementations

It is to be understood that embodiments of the present disclosure can beimplemented in various forms of hardware, software, firmware, specialpurpose processes, or a combination thereof. In some embodiments, thepresent disclosure can be implemented in hardware as anapplication-specific integrated circuit (ASIC), or as a fieldprogrammable gate array (FPGA). In other embodiments, the presentdisclosure can be implemented in software as an application programtangible embodied on a computer readable program storage device. Theapplication program can be uploaded to, and executed by, a machinecomprising any suitable architecture.

FIG. 10 is a block diagram of a system that implements a method forsynthesizing middle non-existing frames from pairs of input frames in agiven video using CNNs according to an embodiment of the disclosure.Referring now to FIG. 10, a computer system 101 for implementing thepresent invention can comprise, inter alia, a processor 102, a memory103 and an input/output (I/O) interface 104. The computer system 101 isgenerally coupled through the I/O interface 104 to a display 105 andvarious input devices 106 such as a mouse and a keyboard. The supportcircuits can include circuits such as cache, power supplies, clockcircuits, and a communication bus. The processor 102 may be a graphicsprocessing unit (GPU), a neural processor or dedicated hardware (HW). AGPU and a neural processor are suitable for running a deep neuralnetwork, while a GPU and dedicated HW are good matches for theregistration and MC-FI modules according to embodiments. The memory 103can include random access memory (RAM), read only memory (ROM), diskdrive, tape drive, etc., or a combinations thereof. The presentdisclosure can be implemented as a routine 107 that is stored in memory103 and executed by the processor 102. As such, the computer system 101is a general purpose computer system that becomes a specific purposecomputer system when executing the routine 107 of the present invention.Alternatively, as described above, embodiments of the present disclosurecan be implemented as an ASIC or FPGA 107 that is in signalcommunication with the processor 102.

The computer system 101 also includes an operating system and microinstruction code. The various processes and functions described hereincan either be part of the micro instruction code or part of theapplication program (or combination thereof) which is executed via theoperating system. In addition, various other peripheral devices can beconnected to the computer platform such as an additional data storagedevice and a printing device.

It is to be further understood that, because some of the constituentsystem components and method steps depicted in the accompanying figurescan be implemented in software, the actual connections between thesystems components (or the process steps) may differ depending upon themanner in which the present invention is programmed. Given the teachingsof the present invention provided herein, one of ordinary skill in therelated art will be able to contemplate these and similarimplementations or configurations of the present invention.

While the present invention has been described in detail with referenceto exemplary embodiments, those skilled in the art will appreciate thatvarious modifications and substitutions can be made thereto withoutdeparting from the spirit and scope of the invention as set forth in theappended claims.

What is claimed is:
 1. A system that uses convolutional neural networks(CNNs) to synthesize middle non-existing frames from pairs of inputframes in a given video, comprising: a coarse convolutional neuralnetwork (CNN) that receives a pair of images acquired at consecutivepoints of time, wherein the pair of images includes a previous image anda next image; a registration module connected to the coarse CNN; arefinement CNN connected to the registration module and the coarse CNN;an adder connected to the refinement CNN and the coarse CNN; and amotion-compensated frame interpolation (MC-FI) module connected to theadder and the coarse CNN, wherein the coarse CNN outputs a previousfeature map and a next feature map from the previous image and the nextimage, a coarse interpolated motion vector field (IMVF) and an occlusionmap from the pair of images, the registration module uses the coarseIMVF to warp the previous and next feature maps to be aligned with pixellocations of the IMVF frame, and outputs a registered previous featuremap and a registered next feature map, the refinement CNN uses theregistered previous feature map and a registered next feature map tocorrect the coarse IMVF, and the adder sums the coarse IMVF with thecorrection to the IMVF and outputs a final IMVF.
 2. The system of claim1, wherein the motion-compensated frame interpolation (MC-FI) modulegenerates an interpolated frame corresponding to a time between the timepoints of the previous frame and the next frame by warping the previousimage and the next image using the final IMVF and performing a weightedblending of the warped previous and next images using occlusion weightsfrom the occlusion map.
 3. The system of claim 1, wherein the coarse CNNreceives the pair of images in a plurality of resolution levels, whereinthe coarse CNN includes a feature extraction sub-network that generatesa pair of feature maps that correspond to each image of the pair ofimages at each level of resolution, an encoder-decoder sub-network thatconcatenates the pair of feature maps at each level of resolution into asingle feature map and processes the single feature map to produce a newfeature map with downscaled spatial resolution, a fusion sub-networkthat merges the new single feature maps at each level of resolution intoa single merged feature map by performing a weighted average of thefeature maps for each level of resolution wherein the weights arelearned in a training phase and differ for each pixel, and an estimationsub-network that outputs horizontal and vertical components of thecoarse IMVF and an occlusion map, and wherein the feature extractionsub-network includes Siamese layers.
 4. The system of claim 3, whereinthe estimation sub-network includes a horizontal sub-module, a verticalsub-module and an occlusion map sub-module, wherein each sub-modulereceives the merged feature map output from the fusion sub-network,wherein the horizontal and vertical sub-modules respectively output ahorizontal probability map and vertical probability map with Sprobability values per pixel in each probability map, wherein eachprobability value represents a probability for a motion vector to be oneof S displacement values for that pixel, wherein the horizontal andvertical sub-modules respectively calculate a first moment of theprobability values for each pixel to determine expected horizontal andvertical components for each pixel, wherein the pairs of expectedhorizontal and vertical components for each pixel comprise the coarseIMVF.
 5. The system of claim 4, wherein the occlusion map sub-moduleoutputs the occlusion map, which comprises per-pixel weights forperforming a weighted average between the previous image and the nextimage.
 6. The system of claim 3, wherein the refinement CNN includes anencoder-decoder sub-network that concatenates the registered previousfeature map and the registered next feature map and outputs a new set offeature maps with spatial resolution resized with respect to a fullresolution of the previous image and the next image, and an estimationsub-network that estimates corrections to the horizontal and verticalcomponents of the coarse IMVF for each block in the registered next andprevious feature maps to output the corrected IMVF.
 7. The system ofclaim 6, wherein the estimation sub-network includes a horizontalsub-module and a vertical sub-module, wherein the horizontal andvertical sub-modules respectively output a horizontal probability mapand vertical probability map with S probability values per pixel in eachprobability map, wherein each probability value represents a probabilityfor a motion vector to be one of S displacement values for that pixel,wherein the horizontal and vertical sub-modules respectively calculate afirst moment of the probability values for each pixel to determineexpected horizontal and vertical components for each pixel, wherein thepairs of expected horizontal and vertical components for each pixelcomprise the correction to the IMVF.
 8. A method of using convolutionalneural networks (CNNs) to synthesize middle non-existing frames frompairs of input frames in a given video, comprising the steps of:receiving a pyramid representation of a pair of consecutive inputframes, wherein the pair of consecutive input frames includes a previousimage and a next image, wherein the pyramid representation includes aplurality of pairs of input frames, each at a different spatialresolution level; generating a pair of feature maps from each resolutionlevel of the pyramid representation and estimating a coarse interpolatedmotion vector field (IMVF) and an occlusion map from each pair offeature maps; registering pairs of feature maps at the same resolutionlevel according to the coarse IMVF and the occlusion map by warping eachfeature map of the pair of feature maps to be aligned with pixellocations of the coarse IMVF and outputting a registered previousfeature map and a registered next feature map; correcting the coarseIMVF using the registered previous feature map and the registered nextfeature map to generate a correction to the IMVF; adding the correctionto the IMVF to the coarse IMVF to generate a refined IMVF; and producinga synthesized middle frame from the pair of consecutive input frames,the refined IMVF and the occlusion map.
 9. The method of claim 8,wherein generating a pair of feature maps comprises generating a pair offeatures maps for each of the plurality of pairs of input frames at eachspatial resolution, wherein each pair of features maps has a spatialresolution downscaled with respect to a resolution of the pair of inputframes; concatenating the feature maps at each resolution level andprocessing the concatenated feature maps to generate a new set offeature maps with downscaled spatial resolution with respect to aresolution of the pair of consecutive input frames, merging the new setof feature maps for all spatial resolution levels into a single mergedfeature map by performing a weighted average of the feature maps foreach level of resolution wherein the weights are learned in a trainingphase and differ for each pixel; and estimating for each block in themerged feature map horizontal and vertical components of the coarseIMVF, and an occlusion map, wherein the occlusion map comprisesper-pixel weights for performing a weighted average between the previousimage and the next image.
 10. The method of claim 9, wherein estimatinghorizontal and vertical components of the coarse IMVF comprises:generating a horizontal probability map and vertical probability mapwith S probability values per pixel in each probability map, whereineach probability value represents a probability for a motion vector tobe one of S displacement values for that pixel, calculating a firstmoment of the probability values for each pixel to determine expectedhorizontal and vertical components for each pixel, wherein the pairs ofexpected horizontal and vertical components for each pixel comprise thecoarse IMVF.
 11. The method of claim 8, wherein the coarse IMVF includestwo 2D maps of horizontal and vertical shifts directed from the IMVF tothe next frame, wherein registering pairs of feature maps comprisesusing the opposite values of these shifts to warp features from theprevious frame to locations of an output frame, and using the horizontaland vertical shifts as is to warp the input features from the next frameto the locations of the output frame.
 12. The method of claim 11,wherein registering pairs of feature maps further comprises: when theregistered previous feature map has an occluded region, replacing theoccluded region in the registered previous feature map with acorresponding region of the registered next feature map, and when theregistered next feature map has an occluded region, replacing theoccluded region in the registered next feature map with a correspondingregion of the registered previous feature map.
 13. The method of claim8, wherein correcting the coarse IMVF comprises: concatenating featuresof the registered previous feature map and the registered next featuremap and processing the concatenated feature maps to generate a new setof feature maps with spatial resolution resized with respect to the fullresolution; and estimating for each block in the coarse IMVF correctionsto the horizontal and vertical components of the coarse IMVF to generatea correction to the IMVF.
 14. The method of claim 8, wherein producing asynthesized middle frame comprises: warping the pair of consecutiveinput frames according to the refined IMVF; and performing a weightedblending of the pair of warped images using the occlusion weights togenerate the synthesized middle frame.
 15. The method of claim 14,further comprising removing blockiness artifacts from motion boundariesby performing a weighted averaging of interpolated pixels in thesynthesized middle frame using estimates from neighboring blocks.
 16. Anon-transitory program storage device readable by a computer, tangiblyembodying a program of instructions executed by the computer to performthe method steps for using convolutional neural networks (CNNs) tosynthesize middle non-existing frames from pairs of input frames in agiven video, comprising the steps of: receiving a pyramid representationof a pair of consecutive input frames, wherein the pair of consecutiveinput frames includes a previous image and a next image, wherein thepyramid representation includes a plurality of pairs of input frames,each at a different spatial resolution level; generating a pair offeature maps from each resolution level of the pyramid representationand estimating a coarse interpolated motion vector field (IMVF) and anocclusion map from each pair of feature maps; registering pairs offeature maps at the same resolution level according to the coarse IMVFand the occlusion map by warping each feature map of the pair of featuremaps to be aligned with pixel locations of the coarse IMVF andoutputting a registered previous feature map and a registered nextfeature map; correcting the coarse IMVF using the registered previousfeature map and the registered next feature map to generate a correctionto the IMVF; adding the correction to the IMVF to the coarse IMVF togenerate a refined IMVF; and producing a synthesized middle frame fromthe pair of consecutive input frames, the refined IMVF and the occlusionmap.
 17. The computer readable program storage device of claim 16,wherein generating a pair of feature maps comprises generating a pair offeatures maps for each of the plurality of pairs of input frames at eachspatial resolution, wherein each pair of features maps has a spatialresolution downscaled with respect to a resolution of the pair of inputframes; concatenating the feature maps at each resolution level andprocessing the concatenated feature maps to generate a new set offeature maps with downscaled spatial resolution with respect to aresolution of the pair of consecutive input frames, merging the new setof feature maps for all spatial resolution levels into a single mergedfeature map by performing a weighted average of the feature maps foreach level of resolution wherein the weights are learned in a trainingphase and differ for each pixel; and estimating for each block in themerged feature map horizontal and vertical components of the coarseIMVF, and an occlusion map, wherein the occlusion map comprisesper-pixel weights for performing a weighted average between the previousimage and the next image, wherein estimating horizontal and verticalcomponents of the coarse IMVF comprises: generating a horizontalprobability map and vertical probability map with S probability valuesper pixel in each probability map, wherein each probability valuerepresents a probability for a motion vector to be one of S displacementvalues for that pixel, calculating a first moment of the probabilityvalues for each pixel to determine expected horizontal and verticalcomponents for each pixel, wherein the pairs of expected horizontal andvertical components for each pixel comprise the coarse IMVF.
 18. Thecomputer readable program storage device of claim 16, wherein the coarseIMVF includes two 2D maps of horizontal and vertical shifts directedfrom the IMVF to the next frame, wherein registering pairs of featuremaps comprises using the opposite values of these shifts to warpfeatures from the previous frame to locations of an output frame, andusing the horizontal and vertical shifts as is to warp the inputfeatures from the next frame to the locations of the output frame, whenthe registered previous feature map has an occluded region, replacingthe occluded region in the registered previous feature map with acorresponding region of the registered next feature map, and when theregistered next feature map has an occluded region, replacing theoccluded region in the registered next feature map with a correspondingregion of the registered previous feature map.
 19. The computer readableprogram storage device of claim 16, wherein correcting the coarse IMVFcomprises: concatenating features of the registered previous feature mapand the registered next feature map and processing the concatenatedfeature maps to generate a new set of feature maps with spatialresolution resized with respect to the full resolution; and estimatingfor each block in the coarse IMVF corrections to the horizontal andvertical components of the coarse IMVF to generate a correction to theIMVF.
 20. The computer readable program storage device of claim 8,wherein producing a synthesized middle frame comprises: warping the pairof consecutive input frames according to the refined IMVF; performing aweighted blending of the pair of warped images using the occlusionweights to generate the synthesized middle frame, and removingblockiness artifacts from motion boundaries by performing a weightedaveraging of interpolated pixels in the synthesized middle frame usingestimates from neighboring blocks.